1. Field of the Invention
The present invention relates to a method for manufacturing a pneumatic tire comprising at least one component made of an elementary semi-finished product of elastomeric material wound in consecutive overlapping turns.
2. Description of the Related Art
A tire comprises a plurality of components, made of elastomeric material possibly incorporating reinforcing elements, assembled together to form a monolithic structure shaped in a toric loop, having a crown portion and two axially opposed side walls each extending radially towards the axis of rotation of the said tire from one edge of the said crown portion. In particular, a radial tire usually comprises, firstly, a carcass containing one or more carcass plies reinforced with reinforcing cords lying in radial planes, that is planes containing the axis of rotation of the tire, and having their ends firmly attached to two annular metal cores, usually known as bead wires, forming the reinforcement of the beads, that is the radially inward edges of the said tire, the function of which is to enable the tire to be fitted to its corresponding mounting rim. On the crown of the said carcass is a thick band of elastomeric material, usually known as the tread band. Between the carcass and the tread band is a reinforcing structure usually known as the belt structure, comprising at least two radially superimposed layers of rubberized fabric containing metal reinforcing cords that run parallel with each other within each layer and lie at an angle with the cords of the adjacent layer, and preferably also a third layer of textile or metal reinforcing cords wound circumferentially, in a radially external position, at least over the edges of the underlying layers.
In addition to possessing the above components, tires intended to be used without an inner tube, also known as “tubeless” tires, are also covered internally with an airtight (that is impermeable to air) layer of elastomeric material. This airtight layer, or “liner” as it is also known, constitutes a further component of the tire of fundamental importance because the correct operation of the tire depends on its integrity over time.
It should be pointed out that, for the purposes of the present description, the term “elastomeric material” means a composition comprising at least one elastomeric polymer and at least one reinforcing filler. This composition preferably also includes additives such as crosslinking agents and/or plasticizers. Owing to the presence of the crosslinking agents, this material can be crosslinked by heating, thus forming the final product.
In conventional tire-building processes, each of the said components is made using the so-called “semi-finished” products, i.e. continuous sheets of elastomeric material prepared separately and in large quantities before the tire itself is made. If composed of the elastomeric material only, these sheets may have a right cross section of variable profile and thickness, whereas if they include reinforcing elements, especially cords, their thickness is largely uniform and they take the name of rubberized fabrics.
The aforesaid “semi-finished” products are collected in special containers, such as reels, and are stored while awaiting assembly on the tire.
The building process that employs these semi-finished products usually consists of a series of successive steps carried out by winding, at each appropriate stage, a particular sheet onto a building drum, cutting (or in some cases pre-cutting) the said sheet into a length approximately equal to the circumference of the drum, and joining the circumferentially opposite ends of the said sheet length directly on the said building drum.
In more recent times particular attention has been given to the search for production methods that would eliminate or at least reduce the preliminary production of semi-finished products that are later used during tire building. For example, as explained in European patent application EP 928 680 by this Applicant, a tire is formed directly on a toroidal support by forming axially overlapping and/or radially superimposed turns of an elementary semi-finished product of suitable dimensions wound on the said support immediately subsequent to its own manufacture. In particular, three different types of elementary semi-finished products are used, viz.: a profile of elastomeric material only, essentially of rectangular section, hereinafter referred to as “strip”; a tape of elastomeric material containing long embedded reinforcing elements, typically textile or metal cords, hereinafter termed “strip-like element”; and rubberized plain metal wires or cords.
In this technology, each component made of elastomeric material only, such as the liner, side walls, fillers and tread band, is formed by winding successive axially adjacent, optionally radially superimposed turns of the said strip onto the said toroidal support.
More precisely, this winding action involves a relative displacement in the axial direction between the said toroidal support, which is turned about an axis coinciding with the axis of rotation of the developing tire, and an output die of an extruder extruding the said strip.